Four new species of Begonia are described from Northern, Peninsular and Southwestern Thailand. Three belong to Begonia sect. Diploclinium (Begonia exposita Phutthai & M.Hughes, Begonia pengchingii Phutthai & M.Hughes and Begonia pseudosubperfoliataPhutthai & M.Hughes), and one (Begonia phutthaii M.Hughes) to Begonia sect. Apterobegonia . All four species are karst limestone endemics. The provisional IUCN status of all the new species is ‘Vulnerable’.

• Begonia exposita Phutthai & M.Hughes sp. nov. Sect. Diploclinium.

The habit resembles that of Begonia putii Craib, B. discreta Craib and B. soluta Craib in terms of the small number of cordate leaves arising directly from a tuber. Begonia putii and B. soluta differ in being smaller plants, .....

Differs from the allied Begonia delicatula in having caducous (not persistent) stipules,
a denser indumentum over all vegetative parts (B. delicatula has glabrous stems,
not densely hairy, and leaves with scattered, not dense hairs). ....

Resembles Begonia subperfoliata Parish ex Kurz in habit but differs in having 5 tepals
in the female flowers (not 2) and glabrous lanceolate bracts (not deeply dissected and
papillose). Also similar to Begonia rabilii Craib in habit but differs in having 5 tepals
in the female flowers (not 4), and leaves with a more deeply crenate margin.